Method for removing ring artifacts from tomograms produced with the aid of a computed tomography unit

Abstract

A method and a computed tomography unit are disclosed, that render it possible in a simple way to remove ring artifacts from tomograms I k , particularly in the case of a fast feed of the recording region per revolution of the recording system of the computed tomography unit by calculating a ring artifact image I k for each tomogram I k . In this procedure, temporary ring artifact images Rt k are firstly calculated for each tomogram I k , and subsequently the final ring artifact image R k is formed for the purpose of correcting the respective tomogram I k by averaging over at least a portion T sub of the temporary ring artifact images Rt k , the temporary ring artifact images Rt k being rotated before averaging in such a way that the ring artifacts present in the temporary artifact images Rt k substantially coincide and their position substantially corresponds to the ring artifacts present in the tomogram I k .

1. A method for removing ring artifacts from tomograms produced with the aid of a computed tomography unit including at least one recording system rotating about a rotation axis, the method comprising:
 reconstructing N tomograms I k  (k=1, . . . , N) at recording positions that are substantially equidistant in the direction of the rotation axis; 
 calculating a temporary ring artifact image Rt k  for each tomogram I k  which has at least one ring artifact present in the respective tomogram I k ; 
 calculating a ring artifact image R k  for each tomogram I k , the respective ring artifact image R k  being formed by averaging over at least a portion T sub  of the temporary ring artifact images Rt k , and the temporary ring artifact images Rt k  being rotated before averaging in such a way that the ring artifacts present in the temporary artifact images Rt k  substantially coincide and their position substantially corresponds to the ring artifacts present in the tomogram I k ; and 
 subtracting the respective ring artifact image R k  from the respective tomogram I k , in each case one result image E k  being produced in which the ring artifact is removed. 
 
     
     
       2. The method as claimed in  claim 1 , wherein the calculation of a temporary ring artifact image Rt k  comprises:
 masking bone and air fractions in the respective tomogram I k  such that a masked image N k  is respectively produced for each tomogram I k , 
 subjecting the respective masked image N k  to high-pass filtering in the radial direction relative to the center of rotation in the masked image N k  such that a high-pass-filtered image H k  is produced for each masked image N k , and 
 artifact threshold value formation in the respective high-pass-filtered image H k  with a negative artifact threshold and a positive artifact threshold, so as to produce for each high-pass-filtered image H k  the temporary ring artifact image Rt k  that has the ring artifacts present in the respective tomogram I k . 
 
     
     
       3. The method as claimed in  claim 2 , wherein the masking of bone and air fractions comprises:
 setting all the image values greater than an upper threshold SWO equal to SWO, and 
 setting all the image values smaller than a lower threshold SWU equal to SWU such that a masked image N k  is produced. 
 
     
     
       4. The method as claimed in  claim 2 , wherein the high-pass filtering comprises:
 carrying out median filtering in radial directions, running through the center of rotation, in the masked image N k  such that a median-filtered image M k  is produced, and 
 subtracting the median-filtered image M k  from the tomogram I k  such that a high-pass-filtered image H k  is produced. 
 
     
     
       5. The method as claimed in  claim 1 , wherein the method includes, before the calculation of the ring artifact image R k :
 carrying out low-pass filtering in the respective temporary ring artifact image Rt k  in the azimuth direction along at least one circular segment that corresponds to a part of a circle seated at the center of rotation and which has a circular segment length matched to the ring artifact. 
 
     
     
       6. The method as claimed in  claim 1 , wherein after the calculation of the ring artifact image R k , the method includes:
 carrying out low-pass filtering in the ring artifact image R k  in the azimuth direction along at least one circular segment that corresponds to a part of a circle seated at the center of rotation and which has a circular segment length matched to the ring artifact. 
 
     
     
       7. The method as claimed in  claim 1 , wherein the calculation of the ring artifact image R k  is performed using the following calculation rule: 
       
         
           
             
               
                 
                   R 
                   k 
                 
                 = 
                 
                   
                     ∑ 
                     
                       l 
                       = 
                       
                         - 
                         T 
                       
                     
                     
                       + 
                       T 
                     
                   
                   ⁢ 
                   
                     
                       w 
                       l 
                     
                     · 
                     
                       
                         Rot 
                         
                           l 
                           · 
                           δ 
                         
                       
                       ⁡ 
                       
                         ( 
                         
                           Rt 
                           
                             l 
                             + 
                             k 
                           
                         
                         ) 
                       
                     
                   
                 
               
               , 
             
           
         
       
       where l is an index traversing the values −T and +T, w l  is a weighting factor dependent on the index l, δ is the angular spacing of ring artifacts between two neighboring tomograms, and Rot is a rotation operator that rotates the temporary ring artifact image Rt l+k  by the angle 1*δ about the center of rotation, the following relationships holding: 
       
         
           
             
               
                 
                   
                     
                       
                         T 
                         sub 
                       
                       = 
                       
                         
                           2 
                           * 
                           T 
                         
                         + 
                         l 
                       
                     
                     , 
                   
                 
                 
                   
                     
                       
                         ∑ 
                         
                           l 
                           = 
                           
                             - 
                             T 
                           
                         
                         T 
                       
                       ⁢ 
                       
                         w 
                         l 
                       
                     
                     = 
                     1 
                   
                 
               
             
           
         
       
       and δ=2*n*d/V, and d corresponding to a difference between the neighboring recording positions and V corresponding to feed of the recording system in the direction of the system axis. 
     
     
       8. The method as claimed in  claim 7 , wherein the following holds for the weighting factors w l :w l =1/T sub . 
     
     
       9. The method as claimed in  claim 1 , wherein at least some of the method steps are carried out in polar coordinates with an origin of coordinates seated at the center of rotation of the images. 
     
     
       10. A computed tomography unit that is designed for carrying out the method as claimed in  claim 1 . 
     
     
       11. The method as claimed in  claim 3 , wherein the high-pass filtering comprises:
 carrying out median filtering in radial directions, running through the center of rotation, in the masked image N k  such that a median-filtered image M k  is produced, and 
 subtracting the median-filtered image M k  from the tomogram I k  such that a high-pass-filtered image H k  is produced. 
 
     
     
       12. The method as claimed in  claim 2 , wherein the method includes, before the calculation of the ring artifact image R k :
 carrying out low-pass filtering in the respective temporary ring artifact image Rt k  in the azimuth direction along at least one circular segment that corresponds to a part of a circle seated at the center of rotation and which has a circular segment length matched to the ring artifact. 
 
     
     
       13. The method as claimed in  claim 3 , wherein the method includes, before the calculation of the ring artifact image R k :
 carrying out low-pass filtering in the respective temporary ring artifact image Rt k  in the azimuth direction along at least one circular segment that corresponds to a part of a circle seated at the center of rotation and which has a circular segment length matched to the ring artifact. 
 
     
     
       14. The method as claimed in  claim 11 , wherein the method includes, before the calculation of the ring artifact image R k :
 carrying out low-pass filtering in the respective temporary ring artifact image Rt k  in the azimuth direction along at least one circular segment that corresponds to a part of a circle seated at the center of rotation and which has a circular segment length matched to the ring artifact. 
 
     
     
       15. The method as claimed in  claim 5 , wherein after the calculation of the ring artifact image R k , the method includes:
 carrying out low-pass filtering in the ring artifact image R k  in the azimuth direction along at least one circular segment that corresponds to a part of a circle seated at the center of rotation and which has a circular segment length matched to the ring artifact. 
 
     
     
       16. The method as claimed in  claim 14 , wherein after the calculation of the ring artifact image R k , the method includes:
 carrying out low-pass filtering in the ring artifact image R k  in the azimuth direction along at least one circular segment that corresponds to a part of a circle seated at the center of rotation and which has a circular segment length matched to the ring artifact. 
 
     
     
       17. The method as claimed in  claim 2 , wherein the calculation of the ring artifact image R k  is performed using the following calculation rule: 
       
         
           
             
               
                 
                   R 
                   k 
                 
                 = 
                 
                   
                     ∑ 
                     
                       l 
                       = 
                       
                         - 
                         T 
                       
                     
                     
                       + 
                       T 
                     
                   
                   ⁢ 
                   
                     
                       w 
                       l 
                     
                     · 
                     
                       
                         Rot 
                         
                           l 
                           · 
                           δ 
                         
                       
                       ⁡ 
                       
                         ( 
                         
                           Rt 
                           
                             l 
                             + 
                             k 
                           
                         
                         ) 
                       
                     
                   
                 
               
               , 
             
           
         
       
       where l is an index traversing the values −T and +T, w l  is a weighting factor dependent on the index l, δ is the angular spacing of ring artifacts between two neighboring tomograms, and Rot is a rotation operator that rotates the temporary ring artifact image Rt l+k  by the angle 1*δ about the center of rotation, the following relationships holding: 
       
         
           
             
               
                 
                   
                     
                       
                         T 
                         sub 
                       
                       = 
                       
                         
                           2 
                           * 
                           T 
                         
                         + 
                         l 
                       
                     
                     , 
                   
                 
                 
                   
                     
                       
                         ∑ 
                         
                           l 
                           = 
                           
                             - 
                             T 
                           
                         
                         T 
                       
                       ⁢ 
                       
                         w 
                         l 
                       
                     
                     = 
                     1 
                   
                 
               
             
           
         
       
       and δ=2*n*d/V, and d corresponding to a difference between the neighboring recording positions and V corresponding to feed of the recording system in the direction of the system axis. 
     
     
       18. The method as claimed in  claim 17 , wherein the following holds for the weighting factors w l :w l =1/T sub . 
     
     
       19. A device for removing ring artifacts from tomograms produced with the aid of a computed tomography unit including at least one recording system rotating about a rotation axis, the device comprising:
 means for reconstructing N tomograms I k  (k=1, . . . , N) at recording positions that are substantially equidistant in the direction of the rotation axis; 
 means for calculating a temporary ring artifact image Rt k  for each tomogram I k  which has at least one ring artifact present in the respective tomogram I k ; 
 means for calculating a ring artifact image R k  for each tomogram I k , the respective ring artifact image R k  being formed by averaging over at least a portion T sub  of the temporary ring artifact images Rt k , and the temporary ring artifact images Rt k  being rotated before averaging in such a way that the ring artifacts present in the temporary artifact images Rt k  substantially coincide and their position substantially corresponds to the ring artifacts present in the tomogram I k ; and 
 means for subtracting the respective ring artifact image R k  from the respective tomogram I k , in each case one result image E k  being produced in which the ring artifact is removed. 
 
     
     
       20. A computer readable medium including program segments for, when executed on a computer, causing the computer to implement the method of  claim 1 .